Both the league and players association have put forward proposals to the other side. One of the key points preventing a deal from getting done, Silver reported, is players' "distrust" of NFL commissioner Roger Goodell after his handling of the New Orleans Saints' "bounty" scandal.

"The long and short of it is, we're not going to agree to a system that doesn't give the player full due-process rights on HGH," NFLPA executive director DeMaurice Smith told Silver. "That's where we started, and that's where we'll end up. We believe in collective bargaining. The fact that the league would rather force us to accept something that's not fair, rather than bargaining for it, is worrisome."

The NFL has agreed to hand off appeals of all positive drug tests to a third-party arbitrator, but union officials want that extended to "other appeals" on incidents such as alcohol-related arrests, marijuana possession, and performance-enhancing drugs. A source also told Silver that there is "no way" players would agree to HGH testing on game days, something for which the NFL has pushed.

The NFLPA's hedging on a potential deal comes amid public pressure from U.S. Reps. Darrell Issa (R-Calif.) and Elijah Cummings (D-Md.), who earlier this year scolded the NFLPA for its "remarkable recalcitrance" on the issue.